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A STUDY OF THE EFFECT OF MOBILITY RATIOS ON PATTERN DISPLACEMENT BEHAVIOR AND STREAMLINES TO INFER PERMEABILITY FIELDS PERMEABILITY MEDIA SUPRI TR 115 By Yuandong Wang Anthony R. Kovscek William E. Brigham December 1998 Work Performed Under Contract No. DE-FG22-96BC14994 Prepared for U.S. Department of Energy Assistant Secretary for Fossil Energy Thomas Reid, Project Manager National Petroleum Technology Office P.O. Box 3628
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A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

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Page 1: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

A STUDY OF THE EFFECT OF MOBILITY RATIOS

ON PATTERN DISPLACEMENT BEHAVIOR AND

STREAMLINES TO INFER PERMEABILITY

FIELDS PERMEABILITY MEDIA

SUPRI TR 115

By

Yuandong Wang

Anthony R. Kovscek

William E. Brigham

December 1998

Work Performed Under Contract No. DE-FG22-96BC14994

Prepared forU.S. Department of Energy

Assistant Secretary for Fossil Energy

Thomas Reid, Project ManagerNational Petroleum Technology Office

P.O. Box 3628

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TABLE OF CONTENTS

List of Figures iii

Acknowledgments iv

Abstract v

Chapter 1 Effect of Mobility Ratio on Pattern Behavior of

A Homogeneous Porous Medium ..................................................... 1

1.1 Introduction .......................................................................................... 1

1.2 Sweep Efficiency ................................................................................. 3

1.3 Simulation Results ............................................................................... 4

1.4 Breakthrough Determination ............................................................... 7

1.5 Discussion of Pattern Behavior ........................................................... 9

1.5.1 Areal Sweep Efficiency ......................................................... 10

1.5.2 Unit Mobility Ratio ............................................................... 13

1.5.3 Five-Spot Pattern, Very Favorable Mobility Ratio ............... 15

1.5.4 Staggered Line Drive, Very Favorable Mobility Ratio ......... 17

1.6 Conclusions ......................................................................................… 19

Appendix A—Effect of Grid-block and Time-step Size .........................……….. 21

Appendix B—Effect of Grid-block Orientation .....................................………... 24

Appendix C—Numerical Dispersion ......................................................……….. 27

Chapter 2 Streamlines to Solve Inverse Problems .......................................... 29

2.1 Definition of the Research Topic ....................................................... 29

2.2 Importance of the Research ............................................................... 35

2.3 Future Work Plan and Expected Results ........................................... 35

Nomenclature 37

References 38

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LIST OF FIGURES

Chapter 1Fig. 1 Dyes et al (1954) Experimental Results ........................................ 2

Fig. 2 Comparison of Simulation Results and

Experimental Results by Dyes et al .....................................…… 6

Fig. 3 Fractional Flow(fw) vs tD at Producer—Determination of

Breakthrough Time ...................................……………………… 8

Fig. 4 Effect of Mobility Ratios and Pattern Geometry

on Areal Sweep Efficiency .....................................….................. 11

Fig. 5 Streamline and Saturation Distribution Unit Mobility Ratio

...............................................................……………………. 14

Fig. 6 Five-Spot Pattern, Very Favorable

Mobility Ratio, 1/M = 20 ...................................................……... 16

Fig. 7 Staggered-Line-Drive Pattern, Very Favorable Mobility Ratio,

1/M =20 ...........................………………………………………. 18

Fig. A Effect of Refining Grid and Increasing Time steps on the

simulation Accuracy ..............................………………..........…. 22

Fig. B.1 Grid Orientation………………………………………………. 24

Fig. B2 Effect of Grid Orientation on Simulation Results .............……. 26

Fig. C Displacing Front ..................................................................……. 28

Chapter 2Fig. 1 Comparison of Permeability Field ......................................……... 31

Fig. 2 Comparison on Breakthrough Curve ....................................…….. 32

Fig. 3 Error vs Iterations ..................................................................……. 34

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ACKNOWLEDGMENTS

This work was supported by the Assistant Secretary for Fossil Energy, Office of

Oil, Gas, and Shale Technologies of the U.S. Department of Energy under contract No.

DE-FG22-96BC14994 to Stanford University. Likewise, the support of the SUPRI-A

Industrial Affiliates is gratefully acknowledged.

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ABSTRACT

In my MS research program I worked on two research topics. I finished the first

research topic and conducted some preliminary research on the second topic. The second

topic is actually a first step toward my Ph.D. program. Below is a brief description of

my work on the two topics.

1. A study of the effect of mobility ratios on pattern displacement behavior

It is well known, for unit mobility ratio, that the areal sweep efficiency of a

staggered line drive pattern is always better than a five-spot pattern. However, this

observation does not hold for very favorable mobility ratios.

I studied the effect of mobility ratios on pattern behavior by the means of

simulation using a streamline simulator. In this report, I present simulation results and,

with the help of streamline and saturation distributions, explain the differences between

displacements with unit and favorable mobility ratios. Simulations compare well with

experiments conducted elsewhere.

Accurate definition of breakthrough time is also discussed for multiphase,

streamline, simulation results. The exact definition of breakthrough is difficult due to

physical dispersion in experiments and numerical dispersion in simulations.

2. Streamline approach to the inverse problem of inferring permeability distribution

It is often useful to infer the permeability field of a porous medium, such as an oil

reservoir or a ground water aquifer from a tracer breakthrough curve at the production

well. There are many approaches to this inverse problem, such as simulated annealing,

sensitivity studies either analytical or numerical, genetic algorithms, and geo-statistical

approaches. Most of these approaches are very time consuming.

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An efficient approach is desirable. Here I propose an alternative approach—an

inverse streamline approach. I have conducted some preliminary work on this topic, to

be described later, and obtained some satisfactory results. These results indicate that this

project is robust and promising. However, there are simplifications in my current study,

such as piston-like displacement, unit mobility ratio and incompressible flow. If these

simplifications can be relaxed, then this can be a very efficient approach to the inverse

problem with broader applications.

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1. EFFECT OF MOBILITY RATIO ON PATTERN BEHAVIOR

OF A HOMOGENEOUS POROUS MEDIUM

1.1 INTRODUCTION

Pattern geometry plays a major role in determining oil recovery during secondary

and enhanced oil recovery operations. Although simulation is an important tool for

design and evaluation, the first step often involves rough calculations based upon areal

sweep efficiencies of displacements in homogeneous, two-dimensional, scaled, physical

models (Dyes et al., 1954; Craig, 1971; Lake, 1989). These results are available as a

function of the displacement pattern and the mobility ratio, M. The mobility ratio is

simply the mobility of the displacing phase over that of the displaced, or resident, phase.

Because it is possible to compute sweep efficiency analytically when the displacing and

displaced phase have the same mobility (Morel-Seytoux, 1966), scaled physical model

results have been verified for unit mobility ratios.

Convincing verification of the non-unit mobility ratio cases does not appear in the

literature. Typical finite difference solutions of the reservoir flow equations suffer from

numerical dispersion, the effects of which are hard to evaluate. Furthermore, the scaled

physical model results at low mobility ratios (M << 1) are provocative. For instance,

Fig. 1a shows that recovery from a five-spot pattern at breakthrough for 1/M greater than

about 6 is virtually 100%, whereas recovery at breakthrough in Fig. 1b for a staggered-

line-drive pattern at an 1/M of 6 is only about 88.5%. This contradicts the common

notion that areal sweep efficiency from a staggered-line-drive pattern is always better

than that from a five-spot pattern.

We use a 3D streamline simulator (Batycky, et al., 1996) to analyze

displacements in five-spot and staggered-line-drive patterns for stable displacements, that

is M less than 1. In the following sections, we present streamline distributions, saturation

distributions, and fractional flow at the producer versus dimensionless time, tD. The

dimensionless time is the pore volumes of displacing fluid injected. With the streamline

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Fig 1 Dyes et al(1954) Experimental Results

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(a) Five Spot Pattern

(b) Staggered Line Drive, d/a = 1

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and saturation distributions at different times, we explain why and at what mobility ratio

the five spot pattern can recover more oil than a staggered line drive pattern.

The streamline calculation method is advantageous in that the results suffer from

much less numerical dispersion than typical finite-difference approximations, but some

dispersion in simulation results is evident. Therefore, we discuss how to treat the

numerical dispersion to obtain accurate estimates of breakthrough times. We discuss the

proper way to calculate fractional flow based on the flow rates at the producer. In

comparing the simulation results with the experimental results of Dyes et al. (1954),

physical dispersion in the experiments is found even though a piston-like displacement

was assumed.

1.2 SWEEP EFFICIENCY

Before proceeding, it is useful to recall the representation of experimental data in

Fig. 1 and the meaning of sweep efficiency. Dyes et al. (1954) used various oils as both

the injected and displaced phases. These hydrocarbons were miscible and they assumed

piston-like displacement. An X-ray shadowgraph technique was used to observe the

position of the displacing front. Areal sweep efficiencies are plotted versus displacable

pore volumes injected for different mobility ratios. In the figure, the x axis is the

reciprocal of mobility ratio. Each curve in the graphs corresponds to a specific tD / ∆ S ,

or displacable pore volume injected. The bottom curves show sweep efficiencies at

breakthrough. It is assumed that the displacement has a piston-like front and there is no

physical dispersion. Likewise, the porous medium is assumed to be perfectly

homogeneous.

For piston-like displacement, the areal sweep efficiency is

EA = AS / AT (1)

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where As is the swept area and AT is the total area. Before and at breakthrough, the

amount of displacing fluid injected is equal to the displaced fluid produced, disregarding

compressibility. Assuming piston-like displacement, injected volume is related to area

swept

VI = AS hφ∆ S (2)

where VI is the volume of displacing fluid injected, h is the thickness of the formation,

and φ is porosity. Hence,

EA = AS / AT = VI / (AT hφ ∆ S) = tD / ∆ S (3)

where tD = V I / (AT hφ) is the pore volume of fluid injected, also commonly called

dimensionless time. For ∆ S =1, EA = tD before and at breakthrough.

After breakthrough,

EA = (VI − VP ) / ( AT hφ ∆ S) (4)

where VP is volume of displacing fluid produced.

1.3 SIMULATION RESULTS

We use a three-dimensional streamline simulator, called 3DSL, written by

Batycky et al (1997) to simulate the displacement for the five spot and staggered line

drive patterns. In the simulations, we set the conditions identical to those in the

experiments and choose relative permeability curves that ensure a piston-like

displacement. The conditions are

1. Homogeneous permeability field, i.e., k is constant.

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2. Straight line relative permeability curves with end point relative permeability of

1, i.e.,

krw = Sw, kro = Sw (5)

Therefore, krw + kro = 1 for any Sw .

4. Mobility is altered by changing viscosity, and the mobility ratio is the

reciprocal of viscosity ratio.

5. We set ∆ S =1 which means that ahead of the displacing front, the displacing

phase saturation is zero, and behind the front, it is unity.

3DSL is very fast compared to conventional finite difference simulators and

exhibits much less numerical dispersion (Batycky et al., 1997; Thiele, et al., 1996). For

our problem, it offers us the streamline distribution which facilitates explanation of

displacement behavior.

I use many pressure solves (time steps) and very fine grids (100 by 100 cells for

the five spot and 140 by 70 cells for the staggered line drive) to ensure converged

simulation results. A grid refinement study (refer to Appendix A for details) showed

these grids to be optimal in that further refinement of the grid did not yield noticeable

changes in breakthrough time, the oil production curve, or displacement patterns. For unit

mobility ratio (M = 1), we actually only need one pressure solve. But for mobility ratios

far from 1, we need many pressure solves. For 1/M = 20, we used up to 1000 pressure

solves to ensure that the results were converged. In the streamline approach, a pressure

solve is accompanied by a re-determination of streamline paths; hence, the flow field.

I also studied the effect of grid orientation on the simulation results (see Appendix

B) and found that the pattern behavior is independent of the grid orientation.

Figure 2 displays areal sweep efficiencies as a function of pore volume injected

for differing mobility ratios, and compares simulation and experimental results (Dyes et

al., 1954). The solid lines are simulation results, solid circles experimental results, and

dotted lines connect circles for ease of viewing. In this figure, we concentrate on only the

favorable mobility ratios, M < 1. We noticed more numerical dispersion for unfavorable

mobility ratio cases, not reported here.

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Fig 2 Comparison of Simulation and Experimental

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(a) Five Spot Pattern

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Figure 3 shows the displacing fluid fractional flow at the producer as a function of

dimensionless time for several mobility ratios. To compute fractional flow from the

numerical data we use a central finite-difference formula rather than backward

differences. The shapes of the fractional flow curves at breakthrough (tD from roughly

0.7 to 1) indicate some numerical dispersion. We expect the fractional flow to increase

sharply rather than gradually at water breakthrough. As expected, the numerical

dispersion decreases as M becomes more favorable (Peaceman, 1977). The most

numerical dispersion occurs for unit mobility ratio, as shown in Fig. 3. We modify the

breakthrough time by trimming the numerical dispersion as will be described next.

1.4 BREAKTHROUGH DETERMINATION

Due to numerical dispersion, injected fluid breaks through earlier at the producer

than it should. However, the numerical dispersion does not have much effect on the late-

time displacing fluid production. The fractional flow versus tD plots shown in Fig. 3

illustrate the early breakthrough caused by numerical dispersion.

To correct for numerical dispersion in breakthrough times and approximate the

breakthrough time more accurately, we use fractional flow data after breakthrough and

extrapolate back to breakthrough time. A least-squares method is used with second order

polynomials:

tD = a + bfw + cfw2 (6)

The data points employed lie between 0.1 < f w < 0.5. The dashed lines in Fig. 3 illustrate

this procedure. All the breakthrough times in Fig 2 are modified using this method.

Numerical dispersion is also related to the number of time steps (i.e., pressure

solves). In the streamline approach applied here, dispersion can be introduced through the

process of mapping the streamline saturation distribution onto the underlying Cartesian

grid used to compute the pressure field5. Hence, for a unit mobility ratio where the

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pressure field does not change, the most accurate results are obtained when a single time

step is used. By performing various single time step simulations, we determine a

breakthrough time of 0.7178 for the five spot with M equal to 1. This is in good

agreement with the analytical solution (Morel-Seytoux, 1966) of 0.7177. Likewise,

Morel-Seytoux analytically and numerically determined breakthrough times for a

staggered-line-drive pattern are both equal to 0.785. With multiple time steps (50)

breakthrough times were sooner, but using the method discussed above, we obtain the

same values for the breakthrough times, at M = 1.

1.5 DISCUSSION OF PATTERN BEHAVIOR

Comparing simulation results and experimental results in Fig. 2, we notice that

breakthrough occurs earlier in the experiments than predicted by the simulations. For the

staggered-line-drive pattern in Fig 2(b), experimental data indicates that EA at

breakthrough is roughly 0.75 for M equal to 1. In the experiments, there is physical

dispersion even though a piston-like front is assumed. In the simulation results presented

in Fig. 2a, the sweep efficiency at breakthrough is 99.7% when 1/M = 20. An almost

negligible area immediately around the producer is not completely swept at breakthrough

due to a very small amount of dispersion in the simulations (refer to Fig. 3, five spot, 1/M

= 20).

After breakthrough, the differences in areal sweep efficiencies between the

experiments and simulations become much smaller (Fig. 2). After breakthrough, the

numerical dispersion consists of only a portion of the displacing fluid produced. As time

increases, this portion decreases and the dispersion has less effect on areal sweep

efficiency. However, the differences between the experimental results and the simulation

results are consistent, i.e. the areal sweep efficiencies of the simulations are generally

higher than those of the experiments. As shown in Fig. 2(a), the simulated breakthrough

curve levels off at large 1 / M with zero slope and does not reach 1 at 1/M = 20. A plot

of displacing front position at late displacement time shows that a very small amount of

numerical dispersion causes slightly earlier breakthrough. Further details are given in

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Appendix C. However, the experimental curve shows EA equal to 1 at 1 / M equal to

about 7.5. We note that in the plot drawn by Dyes et al, the point where the breakthrough

curve hits the EA equal to 1 line is only an extrapolation from other data points.

1.5.1 Areal Sweep Efficiency

Figure 4 plots computed breakthrough, tD , versus the conventional shape factor

d/a for various mobility ratios. The analytical solution for the unit mobility ratio (Morel-

Seytoux, 1966) is also plotted on the same figure for comparison. We find a good match

of the sweep efficiency at breakthrough between the analytical solution and simulation

results.

For unit mobility ratio, Fig. 4 teaches that a staggered-line-drive pattern always

has better areal sweep efficiency than a five-spot pattern. As the staggered-line-drive

pattern becomes longer relative to its width, the displacement pattern approaches linear

flow. High sweep efficiency results.

As the mobility ratio becomes more favorable, the advantage of staggered line

drive on sweep efficiency diminishes. When the mobility ratio decreases to 0.2, the five-

spot pattern becomes better than the staggered-line-drive pattern with d / a = 1. However,

if d / a is increased, the staggered line drive recovery is better than the five spot pattern

for this mobility ratio.

When the mobility ratio decreases to 0.05 or lower (very favorable), the areal

sweep efficiency for the five-spot pattern is very close to 1 at breakthrough. That is,

sweep out is complete at breakthrough. At this mobility ratio, the five spot pattern is as

good as a very long staggered line drive (d / a ≈ 15, almost linear flow), and much

better than the common staggered line drive (d / a = 1). The transition point for five

spot sweep efficiency exceeding that from a staggered line drive is around a mobility

ratio of 0.3. That is to say, if the mobility ratio is higher than 0.3, a staggered line drive

is always better than a five spot. If mobility ratio is lower than 0.3, then the five spot can

be higher in sweep efficiency than a staggered line drive.

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Fig 4 Effect of Mobility Ratios and Patterns On Areal Sweep Efficiency

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1 10Aspect Ratio, d/a

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M = 1.00M = 1.0, AnalyticalM = 0.33M = 0.30M = 0.20M = 0.05

Page 18: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

12

The excellent displacement from a five spot pattern for very favorable mobility

ratios is explained with the help of streamline distributions. Every pair of streamlines

forms a stream tube, and the volumetric flow rate is the same in all the stream tubes. All

of the stream tubes connect with the same injector and producer, and the pressure drop

for all the streamtubes is the same. With the same pressure drop and the same volumetric

flow rate, the flow resistance is the same for all the streamtubes.

For our straight-line relative permeability assumption, we have

Ri = −∆p / q =µkA

dl0

Li∫ (7)

where Ri and Li are the resistance and length of streamtube i , respectively, k is the

homogeneous permeability, and A is the cross sectional area of the streamtube. The

cross-sectional area is

A = hw

where h is the constant thickness of the layer and w is the width of the streamtube.

Resistance in streamtube i is the same as that in streamtube j and thus

µkA

dl0

Li∫ = C (8)

for all the streamtubes at a given time, where C is constant. Moving the constant

parameters k and h to the right hand side, we have

µw

dl0

Li∫ = C (9)

For a piston-like displacing front,

µ µ1

0

2

1 1

wdl

wdl C

l

l

Lfi

fi

i

∫ ∫+ = (10)

Page 19: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

13

where µ1 and µ2 are the constant viscosity of the displacing and displaced fluids,

respectively, and l fi is the distance from the injector to the displacing front.

1.5.2 Unit Mobility Ratio

For a unit mobility ratio, the pressure field remains unchanged throughout the

displacement, and so do the streamlines. The streamline distributions at M = 1 for the

five-spot and staggered-line-drive (d / a = 1) patterns are shown in Fig. 5.

For unit mobility ratio, Eq. (10) is

µ 1

w0

Li∫ dl = C (11)

From Eq. 11, we know that if the ith streamtube is longer than the jth streamtube, then the

average width of the ith streamtube wi is greater to maintain the same resistance and

flow rate. Therefore, the volume of the ith streamtube is larger than the jth streamtube.

The greater the difference in streamtube length, the bigger the difference in streamtube

width, and, when breakthrough happens in the jth streamtube, the front has not

progressed as far in the ith streamtube.

For a five-spot pattern, the longest streamline is that along the boundary, which is

2a. The shortest streamline is the one along the diagonal, at a length of 2 a . The ratio of

the longest streamtube length over the shortest is 2 . Since the width of the longest

streamtube is also 2 times as great as the shortest streamtube, the volume of the

longest streamtube is twice that of the shortest.

However, for a staggered-line-drive pattern with d/a = 1, the ratio of the length

along the boundary (3a/2) over the diagonal( 5a / 2)is 3 / 5 ≈ 1.34 . From the

streamline distribution in Fig. 5, we know that the shortest streamline is longer than

diagonal, and therefore the ratio of the longest streamline over the shortest is less than

1.34. This ratio is about 1.3 and, therefore, less than that ratio for a five spot pattern

which is 1.41. That is to say, the streamlines are more evenly distributed in the staggered

Page 20: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

14

Fig 5 Saturation and Streamline Distributions, Unit Mobility Ratio

(a) Five Spot Pattern

(b) Staggered Line Drive

tD = 0.204

tD = 0.408

tD = 0.612

tD = 0.68

tD = 0.714

tD = 0.748

tD = 1.02

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tD = 0.208

tD = 0.416

tD = 0.624

tD = 0.763

tD = 0.783

tD = 0.797

tD = 0.832

tD = 1.04

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Page 21: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

15

line-drive than in a five-spot pattern. Therefore, when the shortest streamtube breaks

through, a larger portion of the other streamtubes have been swept in a staggered-line-

drive than in a five-spot pattern. When d / a increases, the streamtube length ratio

(longest to shortest) decreases. When breakthrough happens in the shortest streamtube, a

greater portion is swept in the longest streamtube resulting in higher sweep efficiency at

breakthrough.

1.5.3 Five-Spot Pattern, Very Favorable Mobility Ratio

For a favorable mobility ratio ( M < 1), the displacement is stable. For equal

volumetric flow rate streamtubes, Eq. 10 holds. Here, we consider the case of very

favorable mobility ratio, i.e., the displacing fluid viscosity is much higher than that of the

displaced fluid. When the front moves a portion of the way down the streamtube, the

pressure drop is mainly in the displacing phase. That is to say, after a short injection time

(compared to breakthrough), the pressure drop in the displaced phase is negligible.

Therefore, Eq. 10 can be simplified to the following form

1

w0

lti∫ dl ≅ C (12)

In this extreme case, the displacing phase does not feel the producer until it is very close

to it because the pressure drop between the front and the well plays a negligible role in

the displacement. Initially, flow around the injector is radial, because the pattern appears

to be infinite at short times. For example, examine Fig. 6b for tD = 0.583. However, after

the front reaches a corner of the pattern, the no-flow boundary condition along pattern

borders alters the radial flow pattern. Pressure isobars must intersect the no-flow

boundaries at 90o. This constrains the streamlines in the region adjacent to a boundary to

be parallel to it. Because the fluids are incompressible, streamlines cannot terminate. The

flow field in the region near the front transitions from radial to quasi-radial. From the

figure, we also see that streamtubes ahead of the front are narrower along the boundary

than those in the center, which makes the front in the boundary streamtubes move faster

Page 22: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

16

Fig 6 Five Spot Pattern, Very Favorable Mobility Ratio(1/M=20)

(a) Saturation Distribution

(b) Streamline Distribution

tD = 0.583

tD = 0.796

tD = 0.837

tD = 0.878

tD = 0.918

tD = 0.938

tD = 0.969

tD = 0.998

tD = 0.583

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Page 23: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

17

than in the central streamtubes. Little area is unswept and the sweep efficiency at

breakthrough approaches unity.

Similar to before, when the path of displacing fluid in the ith streamtube is longer

than that in the jth streamtube, then the ith streamtube is wider in the swept region to

keep the same resistance (same flow rate and pressure). Therefore, the streamtubes along

the boundary become wider near the corner where the streamtube changes direction.

Streamlines remain smooth. This streamline distribution trend is apparent in Fig. 6b.

In summary, the very favorable mobility ratio conspires with boundary conditions

to determine the way that streamlines evolve, and makes the sweep efficiency at

breakthrough near unity. If the mobility ratio is very favorable, the pressure drop is

mainly in the displacing phase, and it does not feel the well, but is affected by the

boundary.

1.5.4 Staggered Line Drive, Very Favorable Mobility Ratio

For the staggered line drive pattern, the displacement at the beginning is similar to

that in the five-spot pattern. That is, the displacement pattern is radial around the injector

before the front reaches the nearest corner. The differences in displacement behavior

between the two patterns occurs after the front reaches the near corner. For a five spot,

because of the symmetry, the front reaches the two corners at the same time. However,

for a staggered line drive, the front reaches the closest corner first.

After the front passes the near corner, the streamlines evolve in a way similar to

the five spot. The streamtubes along the boundary are wide near the corner but narrow

near the front. This makes the front near the boundary move faster because the

streamtubes are narrower than those in the center of the pattern. Therefore, the front near

the boundary on the near no-flow corner side catches up, and the displacement

approaches linear flow (see the relevant streamline distribution in Fig. 7 at tD = 0.60 and

0.80). If the aspect ratio is large, flow in the center of the pattern must become nearly

linear because the pressure isobars are nearly straight and intersect the pattern boundary

at 90o.

Page 24: A STUDY OF THE EFFECT OF MOBILITY RATIOS ON ...1. A study of the effect of mobility ratios on pattern displacement behavior It is well known, for unit mobility ratio, that the areal

18

Fig 7 Staggered Line Drive, Very Favorable Mobility Ratio(1/M=20)

(a) Saturation Distribution

(b) Streamline Distribution

tD = 0.20

tD = 0.40

tD = 0.60

tD = 0.80

tD = 0.88

tD = 0.90

tD = 0.92

tD = 0.94

tD = 1.0

tD = 0.20

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19

Similar to the discussion above for a five-spot pattern, for a very favorable

mobility ratio, the displacing front is perpendicular to the borders of the pattern both

before and after the front passes the nearest corner. As a result, we see linear

displacement for some time until the front on the near-corner side approaches the

producer. We see these front shapes in Fig. 7.

When the front approaches the producer, the streamtubes narrow due to the

confinement of the pattern boundary and the well. And therefore, with the same flow rate,

the displacing fluid will break through relatively quickly in the streamtubes closest to the

producer. The front on the far no-flow side progresses more slowly. This streamline

distribution does not change greatly as the mobility ratio becomes more favorable.

Sweepout of the pattern is not complete at breakthrough. For instance, a small amount of

the resident fluid remains along the right hand boundary as shown in Fig 7a at tD = 0.94 .

If the length of staggered line drive is increased (increasingd / a ), then the

displacement will approach linear flow and the sweep efficiency will approach unity. The

proportion of unswept area decreases as d / a increases.

1.6 CONCLUSIONS

Pattern performance changes with mobility ratio. For unit mobility ratio,

unfavorable mobility ratios and some favorable mobility ratios (M > 0.3), a staggered-

line-drive pattern has higher areal sweep efficiency than a five-spot pattern. However, for

very favorable mobility ratios (M < 0.3), a five-spot pattern has better sweep efficiency

than a common staggered-line-drive.

The reason for this behavior is the change of streamline and pressure distributions

with mobility ratios. For very favorable mobility ratios, the displacing front is near an

isobar and intersects the pattern boundary at 90o. This causes the fronts at times near

breakthrough to become radial around the producer for a five-spot pattern. This

displacing front shape is due to the symmetry of the five spot pattern.

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For a staggered line drive, the displacing front is also perpendicular to the border

of the pattern. However, because the pattern is not symmetric, sweepout at breakthrough

is not complete. Only in the limit of very large d/a will the areal sweep efficiency

approach 1.

The simulation results are quite close to the analytical solutions for unit mobility

ratio. The results are also very close to the experimental data, Dyes et al. (1954), after

breakthrough at various mobility ratios. We find physical dispersion in the Dyes et al.

experimental results that cause earlier breakthrough time.

We observed some numerical dispersion in our simulation results. For very

favorable mobility ratios, the dispersion is small. We corrected the simulation results by

fitting the fractional flow curve with a second order polynomial to estimate breakthrough

time.

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Appendix A—Effect of Grid-Block and Time-Step Size

The number and size of grid blocks and time steps has a strong effect on

simulation results. The extent of the effect is related to the mobility ratio. This study is

performed on five-spot pattern cases and the results are shown in Fig. A. However, the

conclusions drawn from this study do not lose any generality.

A.1 EFFECT OF TIME-STEP SIZE

Figure A plots the fractional flow at the producer versus time; changes in this

curve versus the number of grid blocks or time step are a convenient measure of the

accuracy of a solution. Comparing the effect of time-step size between M=1 and M=1/20

in Fig A, it is obvious that the effect of time-step size is much stronger in the very

favorable mobility ratio case (M=1/20). For unit mobility ratio (M=1), simulation results

stop changing at 50 time steps. However, for M=1/20, the difference between simulation

results and the converged solution for the same number of time steps is much greater. The

reason is described below.

For unit mobility ratio, the pressure field as well as the streamline distribution

does not change with time. If we map the analytical solution along the streamline, then,

time-step size does not have any effect on the simulation result. However, I mapped the

numerical solution to be consistent with the very favorable mobility rstio cases in which

mapping the analytical solution is not applicable.

As mobility ratio departs from unity, the pressure field changes more intensively.

For a very favorable mobility ratio, we must solve the pressure field enough times to get

a converged solution. We can see that, for M=1/20, the simulation result converges at

100 pressure solves (time steps).

For the calculations presented in the main portion of the report, I used 100 time

steps for unit mobility ratio and 300 time steps for M=1/20. The simulation result is

converged in terms of number of time steps.

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A.2 EFFECT OF GRID-BLOCK AND TIME-STEP SIZE

Figure A also shows that the effect of grid-block size is much stronger in the unit

mobility ratio cases than it is in the very favorable mobility ratio cases. For M=1/20, the

simulation results converge when the number of grid blocks is greater than 10 by 10.

However, for unit mobility ratio, results converge for much finer grid blocks (100 by

100).

In very favorable mobility ratio cases, the simulation result is not sensitive to the

grid-block size. The reason can be the very sharp displacing front for very favorable

displacement.

I used 100 by 100 grid blocks for all the cases in this study, and therefore, the

simulation results are converged in terms of grid-block size.

A.3 CONCLUSION

In the unit mobility ratio cases, simulation results are sensitive to the grid-block

size but not very sensitive to time-step size. In the very favorable mobility ratio cases,

the simulation results are sensitive to grid-block size. For the calculations presented in

the main portion of the report, I used enough number of time steps and grid blocks for a

converged simulation result. Therefore, the results above demonstrate that simulations

are free from effects of the size of grid blocks and time steps.

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Appendix B—Effect of Orientation of Grid-Blocks

We thought that the orientation of the grid-block with respect to the flow direction

may affect the simulation results, especially the streamline distribution. Therefore, I

studied this effect. In the work presented before, the injector and producer are aligned

with the diagonal as shown in Fig. B1a. It was not clear whether the streamline

distribution changes if streamlines align with the grid block (see Fig. B1b). Again, I only

studied the five-spot pattern. However, the conclusion is general for other patterns.

(a) (b)

Fig. B1 Grid Orientation.

I ran the simulation for both cases illustrated in Fig. B1 with time steps and grid-

block size remaining the same.

In 3DSL only rectangular grid blocks can be used, in the rotated grids. Therefore,

I have a bigger domain with the quarter of five-spot pattern sitting in the middle. A very

low permeability was assigned to regions outside the five-spot domain. In doing so, there

is essentially no flow outside the five-spot domain. The no-flow boundary is

approximated by zigzag lines along the boundary. However, as I used many grid

blocks(100 by 100), the boundary is quite smooth.

Because the grid block in the no-flow corner is closed on three sides, it is not

open to flow and it is not filled with the displacing phase when the surrounding blocks in

the five-spot pattern domain are swept by the displacing phase (see Fig. B2). It causes

the breakthrough time to be slightly earlier (about 0.1%). However, it does not make

much difference to the overall pattern behavior.

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There is also some dispersion into the region outside the five-spot domain, as

illustrated in Fig. B2, due to the nonzero permeability in that region. 3DSL does not

work for zero permeability. This dispersion is small and does not affect the pattern

behavior.

Comparing Fig. B2 with Fig. 6, we can easily see that the direction of the grid

does not effect the pattern behavior. Therefore, any difference in the pattern behavior at

different mobility ratios is not due to improper grid orientation.

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Appendix C—Numerical Dispersion

Fig. C shows the position of the displacing front at times near breakthrough (tD =

0.95) for the case of a five-spot pattern and a very favorable mobility ratio, M=1/20. The

solid line plots front location in the streamline closest to the boundary, whereas the

dashed line corresponds the diagonal line between injector and producer. The purpose of

this plot is to study the numerical dispersion for this case, because we think that the

breakthrough time should be unity in the absence of numerical dispersion.

I assumed piston-like displacement in choosing relative permeability curves.

However, because I mapped the numerical solution along the streamlines, this sharp front

may be smeared by numerical dispersion. However, due to the numerical-sharpening

effect for very favorable mobility ratios, the front is still quite sharp.

From Fig. C, it is obvious that the displacing front is very sharp. However, we

can still clearly see that it is not exactly a piston-like displacement due to the numerical

dispersion. The dispersion is very small. If there is no dispersion, the piston-like

displacing front bisects the numerically predicted front. The dimensionless width of the

front is 0.02, and the perfect piston-like front is in the middle. A dimensionless distance

of 0.01 from the front in the simulation result. Therefore, the numerical dispersion

causes 0.01 (tD) earlier breakthrough in the earliest breakthrough streamline. Referring to

Fig. 2(a), the calculated breakthrough tD for this case is over 0.99. Therefore, without

numerical dispersion, there will be perfect displacement for the five-spot pattern at very

favorable mobility ratio.

Fig. C also tells us the front shape along the boundary is almost the same as that

along the diagonal. Therefore, the slanted front is only due to the numerical dispersion,

not the boundary effect.

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Fig. C Displacing FrontFive Spot Pattern, 1/M = 20

30

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2 STREAMLINES TO SOLVE INVERSE PROBLEMS

2.1 DEFINITION OF THE RESEARCH TOPIC

The proposed research is to apply the concept of streamlines to infer permeability

fields based on the tracer breakthrough curve and pressure differences between the

injector and producers. The basic idea of my approach is to adjust the streamlines to

match the reference fractional flow and pressure data, and through streamlines to modify

the underlying permeability field.

Because there is no flow across a streamline, we can represent the flow field by

1D flow streams along the streamlines. Each streamline is associated with a time of flight

which indicates the breakthrough time. In the 3DSL streamline simulator (Batycky et al),

we solve the pressure field at given saturation distribution, obtain the streamline

distribution, then map the Buckley-Leverett solution along the 1D streamlines for a short

time period, and then solve the pressure field again and repeat the process. Because the

pressure equation and saturation equation are decoupled in 3DSL, the simulation is

speeded up significantly.

Conventional reservoir simulation history matching is time consuming because of

the large number of grid blocks. However, if we know the time of flight of the

streamline, then we will know the breakthrough time of an individual streamline.

Therefore, if the breakthrough curve for our computed permeability field does not match

the reference breakthrough curve, then we know along which streamline and in which

way to modify the permeability for a better match.

This approach is described below. To start with this project, I made the

following simplifications:

• Incompressible flow, because the streamline simulator (3DSL by Batycky et al) that I

am using works for incompressible fluid flow only;

• Piston-like displacement;

• Unit mobility ratio, valid for tracer flow study;

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• Two dimensional;

• No constraint of the permeability value or its distribution.

Steps of the Streamline Approach to the Inverse Problem:

1. Given a reference permeability field, use the 3DSL streamline simulator for forward

simulation to obtain the reference water breakthrough curves at producers and

pressure drops between injector and producers.

2. Start from an initial uniform permeability field. Do the same simulation. Check the

match—both breakthrough and pressure drop. If it does not match the reference data,

modify the permeability as in the following steps;

3. Work on the streamlines: Calculate the time of flight for all streamlines using 3DSL

to output the coordinates of the streamlines. Sort the streamlines in the order of time

flight.

4. Compare the computed and the reference breakthrough and pressure drops. For the

breakthrough curve, check where in the curve the difference lies. Relate it to the

corresponding streamline, and then change the permeability value depending on the

difference of the breakthrough curves;

5. Repeat the simulation using the new permeability distribution. This completes one

iteration;

6. Repeat steps 2 to 5 until the match is good enough.

I obtained very promising results with this approach. In the reference field, there

are one injector and two producers. Between the injector and producer there exists a high

permeability channel, and between the injector and producer 2 there is a low permeability

barrier (See Fig. 1). Starting from a homogeneous permeability field, both the

breakthrough and pressure drops are far away from the reference (see Fig. 2). After three

iterations, I obtained a very good match both in pressure drops and breakthrough curves

(Fig. 2), which indicates that this approach converges very fast, much faster than most of

other approaches to this inverse problem.

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Fig. 1 Comparison on Permeability Field

Producer 1

Producer 2

36

Reference Permeability Field

Computed Permeability Field after 3rd Modification

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Fig. 2 Comparison on Breakthrough Curve

37

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Fig. 3 plots the error versus the number of iterations. The error is defined as

[ ]E E Efw i p ii

n prod

= +=∑ , ,

_

∆1

(1)

where Efw is the average absolute error in the fractional flow at different times, and E p i∆ ,

is the relative error of the pressure drops between the injector and producers at producer i.

They are defined below

En

f ffw i w i jC

w i jR

j

n

, , , , ,= −=

∑1

1

(2)

where fw i jC, , and fw i j

R, , are the computed and reference water fractional flow during time

step j at producer i, respectively.

Ep p

pp iiC

iR

iR∆

∆ ∆∆, = −

(3)

where ∆piC and ∆pi

R are calculated and reference pressure drop between the injector and

producer i, respectively.

I normalize the error in pressure by the reference pressure so that it will be within

the range of 0 to 1 and it is of the same scale as the error in fractional flow. Then I can

sum up them up to have a unified error estimation.

The solid line in Fig. 3 (a) is the sum of error at the two producers. The dashed

and dotted line are for each producer, respectively.

Comparing the computed permeability field with the reference field in Fig. 1, we

find that, with this approach, the high permeability channel is retrieved quite accurately.

For the barrier, the resolution is not very good. However, we have good matches for the

breakthrough curve and pressure drop.

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(b)

Fig. 3 Error vs Iterations

38

0.0

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or

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E∆p at Producer 1Efw at Producer 1E∆p at Producer 2Efw at Producer 2

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2.2 IMPORTANCE OF THE RESEARCH

History matching plays an important role in reservoir engineering. It is important

for prediction and data interpretation. In many cases, we have water or tracer

breakthrough and pressure information at the producers. This information contains much

information about the permeability distribution of a reservoir, but it is difficult to infer

this distribution. There are many approaches to this inverse problem. Most approaches

manipulate parameters at the grid-block level that corresponds to conventional simulation

grids. Because there are many cells, the optimization problem is large. By conducting the

optimization at the streamline level, the opportunity for speed improvement is great.

Since we know how each streamline affects water cut, we can speed up the inverse

process tremendously. A more typical approach would be to perturb parameters in a

more random fashion to gauge the effect of the parameter.

2.3 FUTURE WORK PLAN AND EXPECTED RESULTS

There are many simplifications in my current study. Simplification means

limitations in the application. Therefore, my future work will be focused on relaxing the

simplifications to broaden its application. My plan is to

• Release the piston-like displacement and unit mobility ratio assumptions. It is not

difficult to do since I can simply map the Buckley-Leverett solution instead of the

sharp front, and I can map the numerical solution for one-dimensional displacement

instead of the analytical one along the streamline.

• Seek a streamline simulator for compressible flow. Study my current method for

modifying a permeability field to see whether it is still valid for compressible flow.

Also study what modification of the current approach should be made for

compressible flow. Then I can make this approach work for primary recovery and

well-test problems. This will be the most difficult portion and therefore the key part

of this project.

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• Put constraints on the permeability distribution from other information such as

seismic data, and broaden its use to three-dimensional problems. Improve the

method.

• For comparison, develop examples that use a conventional history-matching approach

to infer heterogeneity.

I will continue to work on this topic in my Ph.D. When this project is finished, I

expect that this approach will work for very general history matching purposes.

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NOMENCLATURE

A area, L2

AS area swept, L2

AT total area of the pattern, L2

a distance between like wells (injection or production) in a row, L

d distance between adjacent rows of injection and production wells, L

EA areal sweep efficiency

fw fractional flow of water

h bed thickness, L

k permeability, L2

kro relative permeability of oil

krw relative permeability of water

l length, L

Li length of stream tube i, L

lfi distance of the displacing front from the injector in stream tube i, L

M mobility ratio

p pressure, M/L • T2

q flow rate, L3/T

Ri flow resistance of stream tube i, M/L4 • T

S saturation

Sw water saturation

tD dimensionless time

VI volume of displacing phase injected, L3

w width of a stream tube, L

φ porosity

µ viscosity, M/L• T

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REFERENCES

1. Batycky, R. P., Blunt, M. J., and Thiele, M. R.: “A 3D Streamline-Based ReservoirSimulator” SPE Reservoir Engineering, 12, 246, (1997).

2. Craig, F. F. Jr.: “The Reservoir Engineering Aspect of Water Flooding,” Society ofPetroleum Engineers Monograph, Dallas, TX, 1971.Dyes, A. B., Caudle, B. H., andErickson, R. A., “Oil Production after Breakthrough—as Influenced by MobilityRatio”, Petroleum Transactions, AIME, 201, 27 (1954).

3. Dyes, A.B., Caudle, B.H., and Erickson, R.A.: “Oil Production AfterBreakthrough—as Influenced by Mobility Ratio,” Petroleum Transactions, AIME,201, 27 (1954).

4. Lake, L. W.: Enhanced Oil Recovery. Prentice Hall Inc., New Jersey, 1989.

5. Morel-Seytoux, Hubert J.: “Unit Mobility Ratio Displacement Calculations forPattern Floods in Homogeneous Medium,” SPE J, 6, 217, (1966).

6. Peaceman, D. W., Fundamentals of Numerical Reservoir Simulation. ElsevierScientific Publishing Co., New York, 1977.

7. Thiele, M. R., Batycky, R. P., Blunt, M. J. and Orr Jr, F. M. Jr.: “Simulating Flow inHeterogeneous Systems Using Streamtubes and Streamlines,” SPE. Reser. Eng, 11, 5,(1996).